Apparatus and method for classifying a medium in a cooking chamber

ABSTRACT

A deep fat fryer includes a controller operatively connected to a heating element for controlling the operation thereof to apply heat to a cooking chamber. A temperature sensor is disposed in the cooking chamber for measuring the temperature in the cooking chamber and communicating the same to the controller. The controller selectively operates the heating means to apply heat to the cooking chamber. Utilizing the output of the temperature sensor, the controller automatically determines whether the medium contained in the cooking chamber is water, liquid shortening or solid shortening or whether the cooking chamber is empty. Based on this determination, the controller takes appropriate action including, automatically adjusting its operation to conform to the detected medium

FIELD OF INVENTION

The present invention relates generally to an electronic control system,and more particularly to a control system for use with a heatingapparatus.

The present invention finds advantageous application to a deep fat fryercooking system and will be described with particular reference thereto,it being appreciated that the present invention has broader applicationsand may be used with other heating apparatus such as ovens, pressurecookers, pasta cookers, holding cabinets, furnaces, and water heaters.

BACKGROUND OF THE INVENTION

It has been found that certain food products cook and taste best whenheated at a specific temperature for a set period of time. As a result,restaurants and food service establishments, especially franchise foodchains, have instituted strict criteria for preparation of fried foodproducts. Consequently, restaurants and food service establishments willoften require a deep fat fryer which can operate and maintain specificheating parameters.

Deep fat fryers are widely used in commercial food vendingestablishments, such as fast food restaurants, to heat food products,such as potatoes, fish, chicken, or the like. Accordingly, desirablecharacteristics in a deep fat fryer include rapid heating, withoutovershoot, to an operator selected cooking temperature, accuratemaintenance of the cooking medium temperature to a temperature within anarrow range around the operator selected cooking temperature, minimalwearing of heating element components, and safety features which preventinjury to the operator or damage to the fryer.

Prior art fryers known heretofore typically include a vat for holding acooking medium, temperature selection means for inputting a desiredcooking temperature for a food product, a heating element (e.g., a gasburner or electric element) for heating the cooking medium, temperaturesensing means for sensing the temperature of the cooking medium, and afryer controller for providing overall control of fryer operations. Onesignificant function performed by the fryer controller is control of theheating element.

The heating element is operated in a melt mode, a post-melt mode, anidle mode, a cook mode, and a boil mode. In the melt mode, a "cold"cooking medium is heated at a slow rate to gradually introduce heat tothe cooking medium. Since many cooking mediums are solid at roomtemperature, special care must be taken in operating the fryer to meltthe cooking medium. When solid cooking mediums are quickly heated, hotspots can develop which may damage the fryer and which may scorch thecooking medium, making it unsuitable for use in cooking. Fire or heavysmoking are also possible results of quick heating of said cookingmediums.

The post-melt mode quickly heats the cooking medium to reach an operatorselected setpoint temperature (i.e., cooking temperature) which isoptimum for cooking the food product.

The cooking medium is maintained at a temperature around the operatorselected setpoint temperature in the idle mode. In this mode ofoperation, the fryer awaits introduction of food product into the vat.

In the cook mode, food product is introduced into the cooking medium,and depending on the load size, may cause a drastic drop in thetemperature of the cooking medium. It is during this mode that the foodproduct is cooked.

In a boil mode, the cooking medium is removed from the vat so that acleaning operation can take place. In this respect, water and detergentare introduced into the vat and heated to a predetermined temperature(e.g., approximately 195° F.).

Referring now to the melt mode, prior art systems turn the heatingelement on at constant intervals (i.e., pulse) to gradually introduceheat energy into the cooking medium. Once a predetermined melt-releasetemperature is reached the melt-mode ends, since the cooking medium maynow be quickly heated to the operator selected setpoint temperaturewithout any adverse effects.

With respect to the post-melt mode, the prior art utilizes generally twoapproaches. In the first approach, the heating element is turnedunconditionally on (i.e., full ON), until the temperature of the cookingmedium exceeds a predetermined threshold temperature a predeterminednumber of degrees below the operator selected setpoint temperature. Oncethe cooking medium has exceeded the threshold temperature, the fryercontroller begins pulsing the heating element.

In a second approach to the post-melt mode, the heating element isturned full ON until a predetermined threshold temperature is reached.When the cooking medium reaches this threshold temperature, the heatingelement is turned off, and the internal heat capacity of the fryer isrelied upon to cause the temperature of the cooking medium to continuerising until reaching the operator selected setpoint temperature.

With regard to the idle mode, prior art systems employ several differentapproaches. A first approach is known as ON/OFF control. The heatingelement is either on or off, with no middle state. The heating elementis ON when the temperature of the cooking medium is below the operatorselected setpoint temperature, and OFF when the cooking mediumtemperature is above the setpoint temperature. A second approach isknown as proportional control. The proportioning action occurs within a"proportional band" around the setpoint temperature. Outside this band,the controller functions as an ON/OFF unit, with the heating elementeither fully ON (below the band) or fully OFF (above the band). However,within the band, the heating element is turned on and off for shortintervals, wherein the ratio of ON time to OFF time is varied based uponthe difference between the cooking medium temperature and the setpointtemperature. A third approach is known as PID (proportional withintegral and derivative control). PID combines proportional control withtwo additional adjustments, which help compensate to changes in thesystem. Integral determines how long the cooking medium temperature hasbeen below the setpoint temperature, and derivative determines how fast(i.e., the rate) the cooking medium temperature is changing.

One feature common to many prior art idle mode control strategies isthat they attempt to minimize the peak-to-valley excursions of thecooking medium temperature. The peak-to-valley excursion is the range ofcooking medium temperatures obtained around the setpoint temperature.The maximum temperature establishes the "peak," while the minimumtemperature establishes the "valley." The peak-to-valley excursion ofthe cooking medium temperature is usually minimized by periodicallypulsing the heating element, wherein the pulses have a fixed duty cycle.In this respect, the pulses of heat are intended to add the heatnecessary to balance the heat lost to the surrounding environment.

Referring now to the cook mode, the controller of prior art systemskeeps the heating element unconditionally on during the entire cook modewhen a "full load" has been introduced into the cooking medium.. A fullload is a load of food product which is at or near the maximum load sizefor the fryer. Prior art systems operate in this manner becauseintroduction of food product typically causes a drastic drop in thetemperature of the cooking medium. However, when several cook modes areinitiated successively, there is a build up of stored energy in thefryer. Thus, it is possible to overshoot the operator selected setpointtemperature when an interval of time elapses between a series of cooksand sufficient energy has built up. Furthermore, when a series of cooksare initiated, the bottom temperature (i.e., the minimum temperature ofthe cooking medium reached after the introduction of food product to thecooking medium) will rise with each successive cook. This also occursdue to heat build-up. Thus, each successive cook mode operation will notbe uniform. As noted above, prior art systems operate unconditionally ONthroughout each "full load" cook mode, and consequently do not dissipateany excess heat.

With regard to the boil mode, which is provided to carry out a vatcleaning procedure, prior art systems require the operator to manuallyenter this mode. In this respect, prior art controllers do not sensewhen water has been substituted for the cooking medium in the vat.

There are drawbacks to the operation of the prior art systems in eachmode of operation. With respect to the melt mode, the prior artgenerally operates the same irrespective of the type of cooking medium.However, it would be advantageous to use different rates of heatingdepending on the type of cooking medium being used. In this respect,liquid shortening can accept heat at a faster rate, without any adverseaffects, than can solid shortening. The prior art fails to provide acontroller which can heat the cooking medium more rapidly or bypass themelt mode altogether and begin the post-melt mode at once, when thecooking medium can accept heat at a faster rate. This approach wouldallow for quicker initial heating of the cooking medium. However, it isalso noted that in the case of solid shortening, an unsafe condition canresult from bypassing the melt mode. Possible results of rapid heatinginclude damage to the quality of the shortening itself, heavy smoking orfire. Accordingly, the prior art also fails to provide a controllerwhich can recognize the type of cooking medium in the vat in order toavoid unsafe conditions.

There are also disadvantages to the prior art post-melt mode, whereinthe heating element is continuously on followed by pulses of heat untilit reaches a predetermined threshold temperature below the operatorselected setpoint temperature. In this respect, pulsing might not beneeded or desired depending on the operating conditions and systemparameters. For example, if the temperature of the shortening is closeenough to the setpoint temperature when the continuous heating isterminated, then the internal heat capacity of the fryer may be capableof raising the cooking medium temperature to the setpoint temperature.This phenomenon is commonly referred to as "thermal lag," and can causethe temperature of the cooking medium to arrive at the setpointtemperature without the further application of heat. Furthermore, insome cases, the pulses of heat may not be sufficient to raise thetemperature of the cooking medium to the setpoint temperature. Thisproblem may arise because the duration of each heat pulse is not longenough to overcome heat loss to the surrounding environment.Accordingly, the prior art does not have the ability to adapt topost-melt mode conditions which may differ each time the post-melt modeoccurs.

The alternative prior art approach to the post-melt mode, whereincontinuous heating is followed by a heat cutoff, has similar drawbacks.In this respect, the prior art does not provide for an adjustable heatcutoff temperature. The heat cutoff temperature should vary, since theresulting peak temperature obtained after the heat cutoff cannot beassured each time the post-melt mode occurs. In this respect, the priorart does not adjust the cutoff temperature for different post-melt modeconditions which may be present.

In general, the prior art approaches fail to provide a controller havinga post-melt mode wherein the threshold temperature is modifiable for asubsequent system startup, based upon the peak temperature reachedfollowing the heat cutoff during the proceeding system startup.Furthermore, prior art systems fail to provide a controller whichadjusts the threshold temperature based upon the rate of rise of thecooking medium temperature during the post-melt mode.

The idle mode of prior art systems also has several drawbacks. In thisregard, different system and operating conditions may require more orfewer pulses of heat, consequently frequent control of the heatingelement may be required to maintain the operator selected setpointtemperature during the idle mode. The very nature of the prior artapproach to the idle mode results in many operations of heating elementcomponents, thus reducing the life of these components. In many cases,tight control of the cooking medium temperature is not as beneficial tothe cooking process as is the extension of the life of the componentscomprising the heating element. The prior art fails to provide acontroller that allows the operator to select an acceptable band for thepeak-to-valley temperature excursion, so as to maximize the life ofheating element components.

There are disadvantages to the cook mode of prior art systems as well.In this respect, prior art cook modes fail to compensate for thebuild-up of stored energy, which occurs when successive cookingoperations are initiated. Accordingly, at the end of a series of "fullload" cooks the cooking medium temperature can overshoot the operatorselected setpoint temperature by an unacceptable amount due to thebuild-up of stored energy in the system. In addition, each cook in aseries of cooks will have a different "bottom temperature" as a resultof heat build-up. Therefore, each cook in the series will not beuniform.

The prior art's manual procedure for entering the boil mode poses asafety hazard. In this respect, if the vat is filled with water and thecontroller believes the system is preparing to cook (i.e., begins astart-up cycle), too much heat will be applied to the water, and aboil-over condition could occur. In this respect, damage to the cookingappliance could occur and anyone in close proximity could be injured.

A second aspect of the present invention relates to time compensation,during the cook mode. Time compensation is necessary for convenientoperation of the fryer, since the time for the temperature of a foodproduct itself to reach a predetermined "fully cooked" temperature willvary based upon the quantity of food product in the vat and thetemperature of the cooking medium during the cook mode. In this respect,it would be advantageous to provide time compensation so that anoperator can enter the same cook time each time the same type of foodproduct is being cooked, without concern for the quantity (i.e., loadsize) of food product introduced into the vat and variations in cookingmedium temperature during a cook mode operation.

The present invention addresses the foregoing and other problems, and isdirected to an electronic control system and more specifically to anelectronic control system having a programmable microcontroller andassociated peripherals, for use with heating apparatus, such as fryers,ovens, pressure cookers, pasta cookers, holding cabinets, furnaces, andwater heaters.

SUMMARY OF THE INVENTION

According to the present invention there is provided a temperaturecontrol system for a heating apparatus having input means for inputtinga setpoint temperature, means for heating a medium, means for sensingthe temperature of the medium, and control means for controlling theamount of heat provided to the medium by the means for heating, whereinduring an idle mode of operation the control means causes said means forheating to generate a pulse of heat each time the temperature of themedium changes from a temperature above an idle-ON temperature to atemperature below the idle-ON temperature.

According to another aspect of the present invention there is provided amethod of operating a control system to classify a medium in a cookingchamber as air or non-air comprising the steps of determining a firstmeasured temperature of the medium, storing the first measuredtemperature of the medium, activating a heating element to heat saidmedium for a first predetermined period of time, deactivating saidheating element for a second predetermined period of time, determining asecond measured temperature of the medium, comparing the first measuredtemperature of the medium to said second measured temperature, andclassifying said medium as air if said second measured temperature doesnot exceed said first measured temperature by a predetermined amount.

According to another aspect of the present invention there is provided atemperature control system for a cooking apparatus comprising inputmeans for inputting a setpoint temperature; means for heating a cookingmedium; means for sensing the temperature of the cooking medium; controlmeans for controlling the amount of heat provided to the cooking mediumby said means for heating, wherein during a melt operation the controlmeans causes said means for heating to generate pulses of heat ofuniform duration and duty cycle until a predetermined melt releasetemperature is reached, and thereupon causing said means for heating toprovide continuous heat until a predetermined cutoff temperature isreached, said control means comprising: means for adjusting saidpredetermined cutoff temperature, during subsequent melt operationswherein the predetermined cutoff temperature is adjusted in accordancewith the difference between the setpoint temperature and the peaktemperature obtained after the predetermined cutoff temperature has beenreached.

According to another aspect of the present invention there is provided atemperature control system for a cooking apparatus comprising means forheating a cooking medium; means for sensing the temperature of thecooking medium; means for inputting a cooking medium parameter; andcontrol means for controlling the means for heating, wherein during amelt mode a duty cycle for pulsing the means for heating is determinedin accordance with said cooking medium parameter.

According to another aspect of the present invention there is provided atemperature control system for a cooking apparatus comprising means forheating a cooking medium; means for sensing the temperature of thecooking medium; means for initiating a post-melt mode; means fordetecting whether the temperature of the cooking medium has stoppedrising at a temperature between approximately 200° F. and 220° F., meansfor switching to a boil mode if said means for detecting has detectedthat the temperature of the cooking medium has stopped rising at atemperature between approximately 200° F. and 220° F.

According to another aspect of the present invention there is provided atemperature control system for a cooking apparatus comprising means forheating a cooking medium; means for sensing the temperature of thecooking medium; and control means for controlling the amount of heatprovided to the cooking medium by the means for heating, wherein duringa cook mode said control means dissipates excess heat.

According to another aspect of the present invention there is provided atemperature control system :for a cooking apparatus comprising inputmeans for inputting a desired setpoint temperature; heating means forheating a cooking medium; temperature sensing means for determining thetemperature of the cooking medium; control means for controlling theamount of heat provided by the heating means to the cooking medium,wherein: during a melt mode the heating means providing pulses of heatto the cooking medium, said pulses having uniform duration and dutycycle; during a post-melt mode the heating means providing continuousheat to the cooking medium until a cutoff temperature is reached,thereafter the heating means providing no heat to the cooking medium;said cutoff temperature being modifiable, for subsequent start-ups, inaccordance with the difference between the peak temperature obtainedafter the cutoff temperature is reached and the setpoint temperature,during an idle mode the temperature control means causing the means forheating to generate a single pulse of heat each time the temperature ofthe cooking medium drops below a predetermined temperature.

According to another aspect of the present invention there is provided atemperature control system for a heating apparatus comprising a keypadfor inputting a setpoint temperature; a heating element for heating amedium; temperature sensor for sensing the temperature of the medium;and a control unit for controlling the amount of heat provided to themedium by the heating element, wherein during an idle mode of operationthe control unit causes the heating element to generate a pulse of heateach time the temperature of the medium changes from a temperature abovean idle-ON temperature to a temperature below the idle-ON temperature.

According to another aspect of the present invention there is provided atemperature control system for maintaining a medium at a setpointtemperature comprising means for heating a medium; means for sensing thetemperature of the medium; temperature control means for controlling theamount of heat provided to the cooking medium by the means for heating,wherein said temperature control means comprises means for generating apulse of heat each time the temperature of the medium changes from atemperature above a predetermined pulse temperature to a temperaturebelow the predetermined pulse temperature; means for adjusting theduration of said pulse of heat based upon the difference between peaktemperature resulting from a previous pulse of heat and a valleytemperature resulting from said previous pulse of heat.

According to another aspect of the present invention there is provided atemperature control system for a cooking apparatus comprising inputmeans for inputting a setpoint temperature; means for heating a cookingmedium; means for sensing the temperature of the cooking medium;temperature control means for controlling the amount of heat provided tothe cooking medium by the means for heating, wherein said temperaturecontrol means comprises: means for operating in an idle mode when nocooking is taking place; means for operating in a cook mode when a cookmode is initiated; and means for dissipating excess heat in the cookingmedium during the cook mode prior to the cooking medium reaching thesetpoint temperature.

According to another aspect of the present invention there is provided amethod of heating a cooking medium comprising generating uniform pulsesof heat to the cooking medium, until the cooking medium has reached afirst predetermined temperature; continuously heating the cooking mediumafter the first predetermined temperature is reached and until a cutofftemperature is reached; stop adding heat to the cooking medium after thecutoff temperature is reached and until the cooking medium temperaturechanges from a temperature above an idle-ON temperature to a temperaturebelow the idle-ON temperature; generating a first pulse of heat ofpredetermined duration, and subsequent pulses of heat of a durationdependent upon the peak-to-valley temperature difference resulting froma preceding pulse of heat; and continuously heating the cooking mediumif the temperature of the cooking medium drops below a predeterminedminimum temperature.

According to another aspect of the present invention there is provided atimer controller comprising means for storing time compensation valuesfor corresponding temperatures; means for inputting a setpointtemperature; means for determining an offset value in accordance withsaid setpoint temperature; and means for determining an adjusted timecompensation value for a measured temperature based upon the timecompensation value corresponding to the measured temperature and theoffset value.

According to still another aspect of the present invention there isprovided a method for adjusting a cooking time comprising timecompensation values for corresponding temperatures; inputting a setpointtemperature; determining an offset value in accordance with saidsetpoint temperature; and determining an adjusted time compensationvalue for a measured temperature based upon the offset value and thetime compensation value for the measured temperature.

It is an object of the present invention to provide an electroniccontrol system for a heating apparatus.

It is an object of the present invention to provide an electroniccontrol system as described above which controls temperature andprovides time compensation.

It is another object of the present invention to provide an electroniccontrol system as described above which provides accurate temperaturecontrol with minimized wear of heating element components.

It is another object of the present invention to provide an electroniccontrol system as described above which minimizes the number of timesthe heating element is pulsed during an idle mode and consequentlyminimizes wear on heating element components.

It is another object of the present invention to provide an electroniccontrol system as described above which detects whether the cookingchamber is empty or filled with liquid or solid shortening.

It is another object of the present invention to provide an electroniccontrol system as described above which, depending upon the type ofcooking medium, uses a different duty cycle for pulsing the heatingelement during the melt mode.

It is still another object of the present invention to provide anelectronic control system as described above, which can adjust thecutoff temperature of the post-melt mode so that the temperature of thecooking medium can be rapidly increased to the setpoint temperature,with controlled overshoot.

A still further object of the present invention is to provide anelectronic control system as described above which dissipates excessstored energy in order to prevent unacceptable overshoot of the setpointtemperature during a cook mode and to provide uniform cookingtemperatures.

It is another object of the present invention to provide an electroniccontrol system as described above which recognizes when water is in thevat, and in response automatically enters a boil mode.

A still further object of the present invention is to provide anelectronic control system which provides accurate time compensationduring a cook mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement ofparts, a preferred embodiment of which will be described in detail inthe specification and illustrated in the accompanying drawings whichform a part hereof and wherein:

FIG. 1 is a block diagram of a deep fat fryer arrangement;

FIG. 2 is a block diagram of a fryer controller disclosing a preferredembodiment of the present invention;

FIG. 3 is a time/temperature graph illustrating a melt-mode, post-meltmode and subsequent idle mode;

FIG. 4 is a time/temperature graph illustration of an idle mode in whichthe cooking medium temperature varies symmetrically about a setpointtemperature;

FIG. 5 is a time/temperature graph illustrating a cook mode; and

FIGS. 6A, 6B and 6C show a flow chart for a preferred embodiment of theidle mode.

FIG. 7 shows a flow chart for a preferred heat dissipation algorithm.

FIG. 8 is a graph showing a time compensation curve for a setpointtemperature of 350° F. and a shifted time compensation curve for asetpoint temperature of 330° F.

DETAILED DESCRIPTION

Referring now to the drawings wherein the showing is for the purpose ofillustrating a preferred embodiment of the invention only, and not forthe purpose of limiting same, FIG. 1 shows a block diagram of a deep fatfryer 1 comprising a temperature sensor 2, a gas burner 4, a gas valve6, a vat 8 and a control unit 20. It should be appreciated that othertypes of heating elements can be used in place of the gas burner and gasvalve, including an electric element.

FIG. 2 shows a block diagram of fryer controller 10 illustrating apreferred embodiment of the present invention. It should be appreciatedthat while the preferred embodiment of the present invention relates toa controller for a deep falt fryer, the present invention is alsocontemplated for use with other heating apparatus.

Fryer controller 10 is generally comprised of a control unit 20, amemory subsystem 30, an input subsystem 40, a display subsystem 50, analarm subsystem 60, an output subsystem 70, a temperature sensingsubsystem 80, and a power subsystem 100.

Control unit 20 is comprised of a microcontroller 22 and a watchdogcircuit 24. Microcontroller 22 acts as the central "brain" of the fryercontroller system. In this respect, it manages all temperature controland timing functions. Preferably, microcontroller 22 is an 80C32microcontroller manufactured by Intel Corporation. Watchdog circuit 24is a monitoring circuit which assures, as much as possible, thatmicrocontroller 22 is functioning properly. In this respect, watchdogcircuit 24 is reset by microcontroller 22 every 1/10 of a second. Ifwatchdog circuit 24 fails to receive a reset signal from microcontroller22, the watchdog timer will reset microcontroller 22. It will also beappreciated that control unit 20 can also be configured without watchdogcircuit 24.

Memory subsystem 30 is comprised of a ROM 32, a RAM 34 and an EEPROM 36.ROM 32 provides program instructions to microcontroller 22. RAM 34stores temporary data such as current temperature, time left to cook,etc., while EEPROM 36 stores changeable setup data provided by theoperator or manufacturer. However, unlike RAM 34, EEPROM 36 retainsinformation even when the fryer controller is turned off.

Input subsystem 40 is comprised of keypad decoder circuits 42, a keypad44, a communications port 46 and appliance status input circuits 48.Keypad decoder circuits 42 decode signals generated by keypad 44 andtransmit the decoded signals to microcontroller 22. Keypad decodercircuits 42 reduce the number of input lines required by microcontroller22 to receive data from keypad 44. In addition, decoder circuits 42 alsoprovide electrostatic discharge (ESD) protection. Keypad 44 ispreferably a four-by-four type keypad matrix, which is used by theoperator to input data to microcontroller 22.

Communications port 46 is used to program microcontroller 22 withprogrammable features such as times and temperature. This data istypically stored in EEPROM 36.

Appliance status input circuits 48 receive status signals from externalvalves and switches (e.g., a drain valve interlock, door interlocks, anON/OFF switch, and a gas pressure switch), and feeds the signals tomicrocontroller 22, preferably via optocoupled inputs.

Display subsystem 50 is comprised of display drivers 52 and 56, analphanumeric display 54, and LED status indicators 58. Display driver 52and display driver 56 drive alphanumeric display 54 and LED statusindicators 58, respectively, by conditioning low level outputs frommicrocontroller 22. Alphanumeric display 54 is preferably an 8 or 16character, 14 ; segment LED display, which communicates messages to theoperator from microcontroller 22. LED status indicators 58 are singleLEDs which indicate the status of a system component and inform theoperator whether a particular function has been invoked.

Alarm subsystem 60 is comprised of a variable loudness alarm driver 62and an alarm 64. Variable loudness alarm driver 62 is provided to drivealarm 64 by decoding output from microcontroller 22 to change the volumeof alarm 64. Alarm 64 is used by microcontroller 22 to alert theoperator of the existence of a particular condition. Preferably, alarm64 is comprised of a piezoelectric buzzer.

Output subsystem 70 is comprised of relay driver circuits 72 and relaydriver circuits 74. Relay driver circuits 72 drive relays which turn theheating element on and off. Relay driver circuits 74 drive relays whichcontrol non-critical apparatus of the fryer, such as automated basketlifts.

Temperature sensing subsystem 80 is comprised of a reference voltagesource 82, an analog-to-digital (A/D) converter 84, a conditioningcircuit 86, a probe status circuit 88, a temperature sensor 90 and anoverride circuit 92. Reference voltage source 82 provides a 3.2 voltsteady voltage for calibrating analog circuits. In this respect, A/Dconverter 84 converts the analog voltage of reference voltage source 82to a digital value usable by microcontroller 22. This digital valueprovides a reference value for calibrating temperature sensor 90.Temperature sensor 90 provides temperature readings inside the vat. Thesignal provided by temperature sensor 90 is conditioned by conditioningcircuit 86 and fed to microcontroller 22 through A/D converter 84.Conditioning circuit 86 provides excitation for temperature sensor 90,and provides linearization and amplification of the output signal oftemperature sensor 90. Probe status circuit 88 also receives theconditioned signal from conditioning circuit 86 and determines whethertemperature sensor 90 has failed (e.g., an open probe circuit, or ashorted probe circuit). If probe status circuit 88 determines thattemperature sensor 90 has failed, then override circuit 92 will send asignal to relay driver circuits 72 to turn off the heating element.Likewise, if an excessively high temperature is sensed by temperaturesensor 90, override circuit 92 will send a signal to relay drivercircuits 72 to turn off the heating element. Accordingly, overridecircuit 92 operates independently of microcontroller 22 to preventhazardous conditions.

Power subsystem 100 provides the power required by the components of thefryer controller and is comprised of a power supply 102 and a powerconditioning circuit 104. Power supply 102 supplies power to theelectrical components of the fryer controller, while power conditioningcircuit 104 prevents electrostatic discharge, lightning and otherdestructive energy from reaching the electrical components.

According to the present invention, the fryer controller provides sixdifferent modes of operation with respect to temperature control. Itshould be noted that the names given to each mode have been selectedsolely for the purpose of illustration, and are not intended to limitthe scope of the invention.

The first mode shall be referred to as the "pre-melt mode." In thepre-melt mode the controller determines whether the cooking chamber(i.e., vat) is empty (i.e., filled with air) or filled with liquid orsolid shortening.

The second mode of operation will be referred to as the "melt mode"during which the liquid or solid shortening, which are presumably in thecooking chamber, will be heated at an appropriate rate.

The next mode of operation will be referred to as the "post-melt mode."During this mode, the temperature of the cooking medium is allowed torise quickly towards the operator selected setpoint temperature.Furthermore, during this mode it is determined whether the cookingchamber contains water rather than shortening. If it is determined thatthe cooking chamber is filled with water, the boil mode is automaticallyentered.

The pre-melt mode, melt mode, and post-melt mode, together comprise a"start-up cycle." The mode of operation directly following the start-upcycle will be referred to as the "idle mode." During this mode ofoperation, the temperature of the cooking medium is stabilized aroundthe setpoint temperature. The controller will operate in this mode ofoperation until a cooking operation is initiated by the operator, bypressing a function key on keypad 44. At this time, the controller willenter a mode which will be referred to as a "cook mode." During the cookmode food product introduced into the cooking medium is cooked.

An additional operating mode, which will be referred to as the "boilmode," is used for a cleaning operation, as discussed above.

Pre-melt Mode

The pre-melt mode will begin once the fryer is powered up. Thecontroller will use a default setting stored in memory to determine thetype of melt operation to perform. The type of melt operations consistof a "liquid melt" operation, a "solid melt" operation, or a "no melt"operation. A liquid melt operation signifies that the medium in thecooking chamber is liquid shortening, while a solid melt operationsignifies that the medium in the cooking chamber is solid shortening. A"no melt" operation signifies that the medium in the cooking chamberdoes not require a melt mode and consequently that the melt mode shouldbe skipped. Therefore, if the "no melt" operation is indicated thecontroller will go from the pre-melt mode to the post-melt mode,provided that a hazardous condition has not been detected during thepre-melt mode. The default setting for the melt operation is changeableby the operator after the fryer has been powered up.

During the pre-melt mode the controller will determine whether thecooking chamber is empty or whether it contains liquid or solidshortening.

To determine whether the cooking chamber is empty or filled with liquidshortening, the current temperature of the medium in the cooking chamberis determined and stored in memory. The controller then turns on theheating element for a first predetermined period of time (e.g., a pulseof heat having a duration of approximately 20-40 seconds). Thecontroller then turns off the heating element for a second predeterminedperiod of time (e.g., 30-60 seconds). After the second predeterminedperiod of time has elapsed, the controller once again determines thecurrent temperature of the medium in the cooking chamber. This currenttemperature is compared to the previously stored temperature. If thecurrent temperature exceeds the stored temperature by a predeterminedamount (e.g., 15° F.), then the controller determines that the cookingchamber is empty (i.e., filled with air). Likewise, if the presenttemperature does not exceed the stored temperature by the predeterminedamount, then the controller determines that the cooking chamber containsliquid shortening. During the post-melt mode (described in detailbelow), the controller will determine whether a medium detected asliquid shortening is actually water.

Once the controller has detected that the cooking chamber is empty itcan take corrective action, such as displaying "VAT EMPTY" onalphanumeric display 54, sounding alarm 64, and/or locking upmicrocontroller 22, such that power to microcontroller 22 must beterminated and then restored before it is again operational.

In determining whether the cooking chamber contains solid shortening, itis noted that solid shortening will respond to heat by rising intemperature more quickly than liquid shortening. When heat isdiscontinued, the temperature of solid shortening will drop quickly intemperature, whereas air will drop in temperature more slowly.Accordingly, based upon the foregoing thermal characteristics, in amanner similar to detecting liquid shortening, the controller can detectwhether the medium in the cooking chamber is solid shortening.

In an alternative approach to determining whether the cooking chamber isempty or filled with liquid or solid shortening, the controller willturn on the heating element for a predetermined period of time, longenough to cause a rise in the temperature of the medium in the cookingchamber by a preprogrammed amount. The time for the temperature to risethe preprogrammed amount will be measured. The controller will thenenter a programmed OFF period, during which period it will measure thetime that transpires as the temperature of the medium in the cookingchamber falls by a preprogrammed amount. The total time for thetemperature of the cooking chamber contents to rise and fall will differdepending on the properties of the medium. In this respect, due to theadded factor of the latent heat of fusion, solid shortening will have adifferent rise-fall time than that of liquid shortening. Likewise, anempty cooking chamber will also be discernably different due to theabsence of any matter other than air. Accordingly, this alternativeprocedure allows the controller to identify whether the cooking chamberis empty or filled with liquid or solid shortening prior to entering themelt mode.

If the controller, using the approaches discussed above, determines thatthe medium in the cooking chamber is a different type than the typesignified, the controller can take any of several actions. Among these,the controller can alert the operator that there is a discrepancy andrequire some action from the operator, or the controller can takeindependent action, such as shutting down or changing to the appropriatetype of melt operation.

Melt Mode

During the melt mode, the controller causes the heating element of thefryer to generate pulses of heat of uniform duration and duty cycle,until the cooking medium reaches a predetermined temperature (i.e., the"melt release temperature"). The melt release temperature is typically150° F.-180° F.

If a solid melt operation has been specified, the heating during themelt mode will be very gradual. For example, the pulse of heat may be ONfor eight seconds with a period of 30 seconds. If a liquid meltoperation has been specified, the heating of the cooking medium will beless gradual. For example, the heat pulse may have a duration of 16seconds with a period of 30 seconds. Accordingly, different duty cyclescan be used for different types of cooking mediums. If a "no melt"operation has been specified, then the entire melt mode will be skipped,and the controller will go from the pre-melt mode to the post-melt mode.In this case, the cooking medium will be heated very rapidly.

Once the cooking medium has reached the melt release temperature, themelt mode ends, since the cooking medium will now be an effective heatsink, which protects against isolated hot spots in the fryer and alsoprevents the cooking medium itself from overheating. Accordingly, duringthe melt mode, the temperature of the cooking medium gradually rises.

Post-Melt Mode

As discussed above, once the cooking medium has reached the melt releasetemperature, the controller begins operating in a post-melt mode. Duringthe first portion of the post-melt mode, the heating element iscontinuously on (i.e., full ON). As a safety feature, the controllerwill monitor the temperature of the cooking medium and determine whetherthe temperature of the cooking medium has stopped rising atapproximately 200° F.-220° F. (i.e., the temperature range that isassociated with the boiling point of water over normal altitudevariations). Accordingly, if the temperature of the cooking medium stopsrising within this temperature range and remains stable for apredetermined period of time, the controller will intelligentlyascertain that the medium in the cooking chamber comprises water ratherthan a cooking medium such as shortening. Therefore, the controller canprovide safe operation of the equipment by automatically changing fromthe post-melt mode to a boil mode, which will be described in detailbelow. The controller does this without operator intervention and makesthe change-over known to the operator by visual and/or audible means.

As stated above, the heating element is continuously on during the firstportion of the post-melt cycle. This allows the temperature of thecooking medium to be quickly brought up close to the operator selectedsetpoint temperature. Once the controller determines that thetemperature of the cooking medium has reached a pre-programmed thresholdtemperature (which is below the operator selected setpoint temperature),the heating element is turned off for the remainder of the post-meltmode. This pre-programmed threshold temperature is kept in thecontroller's non-volatile memory and represents a specific temperatureoffset from the operator selected setpoint temperature. When thisthreshold temperature is reached, the heating element is turned off, andthe cooking medium temperature is allowed to coast toward the setpointtemperature. It should also be noted that the threshold temperature isalternatively referred to as the "cutoff temperature" since the heatingelement is "cutoff" at this temperature.

The threshold temperature may be adjusted following each start-up cycle.In this respect, when the temperature of the cooking medium stopsincreasing after the heating element has been turned off at thethreshold temperature, the controller remembers the peak temperaturereached after the turn off and calculates the deviation from theoperator selected setpoint temperature. If the deviation is within apre-programmed acceptable band about the setpoint temperature (e.g., thesetpoint temperature +/-2° F.), the controller will not adjust thethreshold temperature for the subsequent start-up cycle. However, if thedeviation falls outside the pre-programmed acceptable band, thecontroller will adjust accordingly the threshold temperature for thesubsequent start-up cycle. For example, the difference between the peaktemperature and the setpoint temperature is added to or subtracted froma current threshold temperature to obtain an adjusted thresholdtemperature. This adjusted threshold temperature will be used during thesubsequent start-up cycle.

The net effect of the foregoing action is to continuously adapt thethreshold temperature based upon the results obtained during thepreceding start-up cycle. Accordingly, the threshold temperature isadjusted for a subsequent start-up cycle, only if and when the peaktemperature reached after the turn off during the previous start-upcycle falls outside the pre-programmed acceptable band about thesetpoint temperature. In the event that this compensation is such thatit begins to approach an unrealistic condition, the controller willalert the operator that the fryer is malfunctioning.

Alternatively, the threshold temperature can be shifted up or down bysome amount proportional to the rate of rise of the cooking mediumtemperature. For example, if the controller has in its memory a valuefor the nominal rate of temperature rise and the actual measured valueis less than this nominal value, then the controller can move thethreshold temperature closer to the setpoint temperature by an amountproportional to the difference in the actual measured rate from that ofthe nominal. The opposite would occur in the case where the actualmeasured rate is greater than the nominal, although in practice theformer is more often the case.

If the peak temperature reached, after the heating element is turnedoff, exceeds the setpoint temperature, the heating element will remainoff until the temperature falls to the setpoint temperature. However,the idle mode, which is explained in detail below, will begin once thepeak temperature has been reached.

If the peak temperature reached is below the setpoint temperature then amove-to-idle algorithm becomes operational. The move-to-idle algorithmprovides a single pulse of heat which has a long enough duration (e.g.,40 seconds) to cause the temperature of the cooking medium to rise abovethe setpoint temperature. The heating element is then turned off andremains off until the temperature of the cooking medium falls to thesetpoint temperature. As noted above, the idle mode begins once the peaktemperature above the setpoint temperature is reached.

Idle Mode

During the idle mode, the controller causes the heating element to keepthe temperature of the cooking medium within a range of temperatures(i.e., a control band defined by Tmin and Tmax) around the setpointtemperature. In a preferred embodiment of the present invention, thetemperature control algorithm for the idle mode gives the operator theability to program an acceptable temperature range about the setpointtemperature for the cooking medium temperature.

In an alternative embodiment of the present invention, the time intervalbetween the start of any two pulses of heat may be programmed (e.g., bythe operator), and thereby obtain whatever peak-to-valley excursionsthat will occur as a result thereof. The controller will automaticallyadapt to the physical system that it is controlling, forcing whatevertemperature excursions that are necessary so as to achieve the correctinterval between pulses of heat. In this respect, if the time intervalbetween two pulses of heat exceeds the programmed time interval, thenthe duration of the pulse is decreased. If the time interval between twopulses of heat is less than the programmed time interval, then theduration of the pulse is increased.

Accordingly, the controller either measures the peak-to-valley excursionof the cooking medium temperature about the setpoint temperature andthen makes a correction to the duration of the heat pulse, or thecontroller times the interval between the start of pulses of heat andthen makes a correction to the duration of the heat pulse. In eithercase, the correction to the duration of the heat pulse is made only onceper cycle each time the pulse occurs.

While initially the heating element will be pulsed when the cookingmedium temperature crosses the setpoint temperature (i.e., crosses froma temperature above the setpoint temperature to a temperature below thesetpoint temperature), once the peak-to-valley temperature swings arestabilized, the asymmetry about the setpoint temperature is evaluated.The temperature at which the heating element is pulsed is then adjusted(i.e., lowered or raised) in order to obtain peak-to-valley temperatureexcursions which are symmetrical about the operator selected setpointtemperature. The adjusted temperature is referred to as the "adjustedidle ON setpoint temperature." Accordingly, the heating element will bepulsed when the cooking medium temperature crosses the adjusted idle ONsetpoint temperature (i.e, crosses from a temperature above the adjustedidle ON setpoint temperature to a temperature below the adjusted idle ONsetpoint temperature).

If the temperature at which the heat pulse occurs is not adjusted forsymmetry, there would be a tendency for the peak-to-valley temperatureexcursions to occur asymmetrically about the operator selected setpointtemperature, thus giving an appearance of operation at a temperatureother than that selected by the operator.

Cook Mode

It has been observed that if the heating element is turned on andremains on during a cook mode for a long period of time, or if a seriesof cooks are initiated, one-after-another, that there is a residual heatbuild-up in the system. This residual heat build-up will often result inserious overshoots of the setpoint temperature. Not only is the recoveryback to the setpoint temperature affected, but the thermalcharacteristics of each successive cook can also be altered, thusresulting in unacceptable changes in the quality of the cooked foodproduct. In this respect, the bottom temperature reached after a load offood product is introduced into the cooking chamber will rise assuccessive cooks are initiated. Accordingly, the controller of thepresent invention will dissipate residual heat during the cook mode.

In general, during the cook mode, the heating element will becontinuously on, as long as the temperature of the cooking mediumremains below a predetermined temperature that is below the setpointtemperature (e.g., a temperature 10° F. below the setpoint temperature).This predetermined temperature is referred to as the "TURN-OFFtemperature." However, as noted above, the controller will dissipatebuilt-up heat in the system. In this respect, the controller willtemporarily turn off the heating element sometime during the cook modeat a temperature below the TURN-OFF temperature. Accordingly, theheating element is temporarily turned off at some time following a risein the cooking medium temperature, after the introduction of foodproduct, but before reaching the TURN-OFF temperature. In a preferredembodiment of the invention the controller is turned off as soon as thecooking medium temperature begins to rise (following introduction offood product to the cooking chamber) and is turned back on as soon asthe temperature of the cooking medium begins to fall. In an alternativeembodiment of the present invention, the heating element is turned offwhen the cooking medium temperature has risen to a predeterminedtemperature that is below the TURN-OFF temperature and remains off untilthe cooking medium temperature falls by a predetermined amount.Temporarily turning the heat off for some period of time during the cookmode provides the heat dissipation necessary to prevent overshoot andprovide uniform cooking.

Following the dissipation of heat, the heating element will continue tobe on until the temperature of the cooking medium reaches thepredetermined TURN-OFF temperature which is below the setpointtemperature. The TURN-OFF temperature is determined to allow thetemperature of the cooking medium to coast above the setpointtemperature (and thus re-enter the idle mode) without risking seriousovershoot of the setpoint temperature. It should be noted that even if acook timer elapses, the heating element will remain on until thepredetermined TURN-OFF temperature is reached. Once a peak temperatureabove the setpoint temperature is reached, the controller will re-enterthe idle mode. If the cooking medium temperature is unable to coastabove the setpoint temperature, the heating element will be pulsed for aduration sufficient for the cooking medium to exceed the setpointtemperature.

It will also be appreciated that the present invention can alternativelybe configured without the heat dissipation feature of the cook mode.

The cook mode is initiated by the operator by selecting a "product key"on keypad 44 corresponding to a particular food product (e.g., frozenfrench fries). A cooking time is pre-stored in memory for each foodproduct option.

An alternative embodiment of the present invention includes an"instant-on" feature. Once a product key is depressed to begin the cookmode, the controller immediately turns the heating element on, withoutregard to the temperature of the cooking medium. After a period ofapproximately 15 seconds, the controller will evaluate whether thetemperature has gone up or fallen. If the temperature has gone up, theheating element is turned off, whereas if the temperature has fallen,the heating element will remain on (i.e., continuously ON).

It will also be appreciated that automated or manual basket lifts may beused to introduce food product into the cooking medium during the cookmode. In this respect, initiation of the cook mode by the operator canbe used to signal the automated basket lifts to drop into the cookingmedium.

Boil Mode

The boil mode is used to periodically clean the cooking chamber of theappliance. Cleaning is performed by filling the cooking chamber withwater and detergents and then heating the solution to a predeterminedboil mode temperature (e.g., approximately 195° F.). However, ahazardous boil-over condition can occur if a melt mode begins whilewater is in the cooking chamber. In this respect, after the melt releasetemperature has passed and continuous heat is applied, the temperatureof the cooking chamber contents will exceed the boiling point of water.Accordingly, a boil-over condition can result in damage to the cookingappliance and possible injury to anyone in close proximity. Therefore,the controller of the present invention will automatically change from amelt mode to a boil mode when water is detected in the cooking chamber.

DETAILED OPERATION

Detailed operation of the controller will now be explained withreference to FIGS. 3-8.

With reference to FIG. 3, there is shown a time/temperature graph of atypical melt mode, post-melt mode and idle mode. During the melt mode,the heating element is pulsed at a constant rate, as shown at portion302 of the heating element signal. Accordingly, the temperature of thecooking medium will gradually rise, as shown at portion 202 of thecooking medium temperature line 200. Once the temperature of the cookingmedium reaches the melt release temperature of approximately 150°F.-180° F., the post-melt mode begins (see time reference A), and theheating element operates continuously on (i.e., full ON) until amodifiable cutoff temperature is reached. Accordingly, the heatingelement remains unconditionally ON until the modifiable cutofftemperature is reached, as indicated at portion 304 of the heatingelement signal. This allows the temperature of the cooking medium toquickly rise to a temperature close to the setpoint temperature selectedby the operator. This is shown by portion 204 of cooking mediumtemperature line 200. The cutoff temperature is generally 25°-35° F.below the operator's selected setpoint temperature. When the temperatureof the cooking medium reaches the cutoff temperature, the heatingelement is turned OFF, as shown at time reference B. With the heatingelement turned OFF, the temperature of the cooking medium will continueto rise (due to residual heat) until it reaches a peak temperature nearthe setpoint temperature. The idle mode begins when the peak temperatureis reached, as shown at time reference C.

Referring now to the idle mode, the heating element remains OFF, and thecooking medium temperature is allowed to coast downward until reachingthe setpoint temperature. Once the cooking medium temperature changesfrom a temperature above the setpoint temperature to a temperature belowthe setpoint temperature, the heating element is pulsed (i.e., turned ONfor a predetermined period of time, as shown at time reference D).Portion 306 of the heating element signal illustrates a first pulse ofheat during the idle mode. This initial pulse is of predeterminedduration. A control band is established above and below the setpointtemperature from, for example, 2° F. below the setpoint temperature to2° F. above the setpoint temperature. These temperatures are referred toas Tmin and Tmax respectively. At temperatures above Tmax, the heatingelement is unconditionally OFF, whereas at temperatures below Tmin, theheating element is unconditionally ON. As the temperature of the cookingmedium changes from a temperature above the setpoint temperature to atemperature below the setpoint temperature, the heating element is againpulsed.

As noted above, the first pulse generated during the idle mode will beof a predetermined duration. The duration of subsequent heat pulses willbe varied based upon the peak-to-valley temperature difference resultingfrom the previous heat pulse. If the peak-to-valley temperature swing(i.e., difference) exceeds a threshold value, for example, 4° F., thenthe duration of the pulse is decremented. In other words, the controllermonitors the highest temperature (i.e., peak) obtained as a result of aheat pulse against the lowest temperature (i.e., valley) reached beforea subsequent heat pulse causes the temperature of the cooking medium torise. Thus, the duration of each successive pulse is based upon thepeak-to-valley temperature swing generated by the previous pulse. If thepeak-to-valley temperature swing is less than what is desired, then theduration of the heat pulse is incremented.

Once the peak-to-valley temperature swings are stabilized, the asymmetryabout the setpoint temperature is evaluated, and the temperature atwhich the heating element is pulsed is lowered or raised in order toobtain a peak-to-valley temperature swing approximately symmetricalabout the setpoint temperature. Accordingly, the cooking mediumtemperature waveform is essentially shifted by adjusting the temperatureat which the heating element is pulsed. As noted above, the adjustedtemperature is referred to as the "adjusted idle ON setpointtemperature."

Referring now to FIG. 4, there is shown a time/temperature graphillustrating an idle mode having a symmetrical temperature swing aboutthe operator selected setpoint temperature. A heat pulse 502 occurs asthe temperature of the cooking medium (see cooking medium temperatureline 400) moves from a temperature above the adjusted idle ON setpointtemperature to a temperature below the adjusted idle ON setpointtemperature. In the example shown in FIG. 4, the adjusted idle ONsetpoint temperature is below the operator selected setpoint temperaturein order to provide a symmetrical temperature swing about the operatorselected setpoint temperature. Another heat pulse 504 occurs as thetemperature of the cooking medium again changes from a temperature abovethe adjusted idle ON setpoint temperature to a temperature below theadjusted idle ON setpoint temperature. The system will remain in thissymmetrical state until a cooking operation is initiated by theoperator, and the cook mode is entered.

Referring now to FIG. 5, a time/temperature graph of a cook mode isshown. The controller operates in the idle mode until a cook mode isinitiated by the operator. A cook mode is initiated by the operator attime reference A, which causes the controller to turn the heatingelement on, as indicated at portion 802 of the heating element signal.In response to the introduction of food product, cooking mediumtemperature line 600 plunges quickly beginning at time reference B. Astemperature line 600 recovers and begins to rise, the heating element isturned off (as shown at time reference C). Accordingly, excess heat isdissipated. Once the cooking medium begins to drop, the heating elementis turned on (as shown at time reference D) and will remain on until theTURN-OFF temperature is reached at time reference E. This is shown byportion 804 of the heating element signal. Once a peak temperature abovethe setpoint temperature is reached, the controller will re-enter theidle mode.

The controller system of the present invention also includes a boil modeduring which the controller sustains the temperature at a predeterminedboil mode temperature (e.g., approximately 195° F.). As discussed above,the present invention includes a safety feature in the event that wateris in the cooking chamber, and a start-up cycle has been initiated. Inthis respect, during the post-melt mode, the controller monitors thetemperature of the cooking medium in the cooking chamber and determineswhether the temperature has stopped rising at approximately 200°-220° F.(i.e., the temperature range that is associated with the boiling pointof water over normal altitude variations). If the temperature hasstopped rising, then it is determined that water is in the cookingchamber rather than shortening. Accordingly, the system willautomatically transfer from the post-melt mode to the boil mode and dropand control the operating temperature to the predetermined boil modetemperature. This feature prevents the possibility of violent boiling ofwater during an intended cleaning procedure.

Referring now to FIGS. 6A, 6B and 6C, there is shown a flow chart for apreferred embodiment of the idle mode. Table 1 set forth below definesthe terms used in the flow charts shown in FIGS. 6A, 6B, 6C and 7:

                  TABLE 1                                                         ______________________________________                                        Term       Definition                                                         ______________________________________                                        T.sub.MEDIUM                                                                             Temperature of the cooking medium                                  T.sub.SETPOINT                                                                           Setpoint temperature                                               T.sub.BAND One half the temperature control band                              T.sub.MAXIMUM                                                                            Maximum cooking medium temperature                                            reached during a cycle of the idle mode                            T.sub.MINIMUM                                                                            Minimum cooking medium temperature                                            reached during a cycle of the idle mode                            TIMEON     Time duration of a pulse of heat                                   MINTIME    Minimum time duration for the pulse of heat                        MAXTIME    Maximum time duration for the pulse of heat                        TEMPON     Temperature at which the pulse of heat is                                     initiated                                                          MAXTEMPON  Maximum temperature at which the pulse of                                     heat is initiated                                                  MINTEMPON  Minimum temperature at which the pulse of                                     heat is initiated                                                  ______________________________________                                    

With reference to FIG. 6A, the idle mode begins by determining whetherthe temperature of the cooking medium is falling (Step 902). If not, itdetermines whether the duration of the pulse has elapsed (Step 904). Ifthe duration of the pulse has elapsed, the heat is turned off (Step 906)and another cycle of the idle mode algorithm is begun.

If the temperature of the cooking medium is falling, it is determinedwhether the temperature has crossed the setpoint temperature (Step 908).If so, then it is determined whether the maximum temperature reached isequal to the maximum temperature for the selected control band (Step910). If this is the case, then the pulse duration is correct for theselected control band and the algorithm continues to FIG. 6B which isdiscussed below. If the maximum temperature reached is not equal to themaximum temperature for the selected control band, then the duration ofthe pulse is either decreased (Steps 914, 916 and 918) or increased(Steps 920, 922 and 924). The duration of the pulse will be set betweena minimum time duration (MINTIME) and a maximum time duration (MAXTIME).

Referring now to FIG. 6B, this portion of the idle algorithm will forcethe peak-to-valley temperature swing of the cooking medium temperatureto be symmetrical about the setpoint temperature. In this respect, it isdetermined whether the minimum temperature reached is equal to theminimum temperature for the selected control band (Step 926). If so, thepeak-to-valley temperature swing is symmetrical. Accordingly, thevariables are reset (Step 942) and the heat is turned on for thecomputed duration (Step 944). If the minimum temperature reached is notequal to the minimum temperature for the selected control band (Step928) then the temperature at which the pulse of heat is initiated isincreased (Steps 930, 932 and 934) or decreased (Steps 936, 938 and940). The temperature at which the pulse of heat is initiated will beset between a minimum temperature (MINTEMPON) and a maximum temperature(MAXTEMPON).

It should be noted that in a preferred embodiment of the presentinvention, before performing step 928, it is determined whether thepeak-to-valley temperature swings have stabilized about the setpointtemperature. If the swings have stabilized then the algorithm proceedswith step 928. If the swings have not stabilized then the algorithmproceeds with step 942. In this manner, the idle algorithm will notproceed with forcing the peak-to-valley temperature swings to besymmetrical about the setpoint temperature until the peak-to-valleytemperature swings have stabilized.

Referring to FIG. 6C, there is shown an algorithm for determining theminimum and maximum temperature excursions of the cooking mediumtemperature. This algorithm will be executed each time a temperaturereading of the cooking medium is taken. The temperature medium of thecooking medium is obtained (Step 950). If the cooking medium temperatureexceeds the temperature at which a pulse of heat is initiated (Step952), then it is determined whether a new maximum temperature has beenreached (Step 954). If so, this temperature is saved as the new maximumtemperature (Step 956).

If the temperature of the cooking medium is less than or equal to thetemperature at which a pulse of heat is initiated (Step 952), then it isdetermined whether a new minimum temperature has been obtained (Step960). If so, this temperature is saved as the new minimum temperature(Step 962).

FIG. 7 discloses a general heat dissipation algorithm for use during thecook mode. It is determined whether a heat dissipate flag has been set(Step 984). If not, the algorithm determines whether the temperature ofthe cooking medium is rising (Step 986). If the temperature of thecooking medium is rising, then the heat is turned off (Step 988) and theheat dissipate flag is set (Step 990).

If it is determined that the heat dissipate flag is set then it isdetermined whether the temperature of the cooking medium is falling(Step 992). If so, the heat is turned on (Step 994) and the normal cookmode is resumed (Step 996).

In summary, the heat dissipation algorithm of FIG. 7 will turn off theheat during the cook mode as soon as the temperature of the cookingmedium begins to rise. The heat will remain off until the temperature ofthe cooking medium begins to fall. Once it begins to fall, the heat isturned back on, and a normal cook mode resumes. It will be appreciatedthat alternatively, the heat can be dissipated at a time later in thecook mode. For instance, the heat could be turned off after the cookingmedium temperature has risen to a predetermined temperature and resumeheating after the temperature of the cooking medium has dropped apredetermined number of degrees.

It should be noted that the foregoing temperature control operations canbe enhanced by saving the operational parameters each time the fryer isused. In this respect, a comparison can be made between currentoperational parameters and previously saved operational parameters,which are used as default or starting values upon system power up. Ifthe default and current values differ by a significant and programmableamount, then the controller will save the current values in protectedmemory for use as the new default values. In this manner, the controllercan adapt itself to changing conditions to achieve a steady-statecondition in the fastest possible time.

Cooking Time Compensation

A second aspect of the present invention relates to time compensationduring the cook mode. In this respect, the preferred embodiment of thepresent invention employs a time compensation curve which relatestemperature to a time compensation factor. Time compensation factors fora time compensation curve having a one-for-one time compensation factorat a temperature of 350° F. are stored in memory. Each time compensationfactor is stored in an individual memory location. The temperaturecorresponding to each time compensation factor is used to provide theaddress of the memory location containing the corresponding timecompensation factor. Accordingly, the temperature acts as a pointer to astorage location in memory containing the time compensation factorcorresponding to that temperature. The stored compensation factors areused as reference data for determining time compensation factors atvarious measured cook medium temperatures. It should be noted that thetime compensating factors of FIG. 8 have been chosen solely for thepurpose of illustration. Accordingly, other time compensation factorsfrom different time compensation curves can be utilized with similarresults.

In response to selection of a setpoint temperature by an operator, ashift factor is calculated by subtracting the time compensation factorcorresponding to 350° F. from the time compensation factor correspondingto the operator selected setpoint temperature. This shift factor iscalculated only once for each setpoint temperature selected. Once a cookis initiated, the time compensation factor corresponding to the measuredcooking medium temperature is retrieved from the memory, and theretrieved time compensation factor is adjusted by the previouslycalculated shift factor (i.e., the shift factor is subtracted from theretrieved time compensation factor). The result of this calculationprovides the adjusted time compensation factor used to adjust the actualcooking time. FIG. 8 shows a time compensation curve for a setpointtemperature of 350° F. (Curve A) and a shifted time compensation curvefor a setpoint temperature of 330° F. (Curve B). A time compensationfactor of 1.0 signifies that each second counted by the controller willelapse in one second (i.e., one-for-one time compensation), whereas atime compensation factor of 1.5 signifies that each second counted bythe controller will elapse in 1.5 seconds.

One alternative to the foregoing time compensation scheme is to store inmemory several sets of time compensation factors from numerous timecompensation curves, each having a one-for-one time compensation factorat different temperatures. The set of time compensation factorscorresponding most closely with the operator selected setpointtemperature is used for the cook. Accordingly, no shift factor need becalculated. One drawback to this approach is that it requires arelatively large amount of memory.

Another alternative time compensation scheme is to store a set of timecompensation factors for a single time compensation curve, but notcalculate a shift factor or adjusted time compensation factor. Onedrawback to this approach is that when the setpoint temperature selectedby the operator does not correspond with the stored set of timecompensation factors, the operator-programmed cook time will be alteredby the time compensation factor associated with the setpoint temperatureinput by the operator. Accordingly, this approach can lead to confusionof the operator with respect to the desired operator input cook time.

In summary, the present invention provides a system which maximizes thelife of heating element components. These elements include relays,contactors, and in the case of gas fired appliances, ignitors and gasvalves. Accordingly, the temperature control algorithm for the idle modeof the present invention is particularly well suited for use with othertypes of heating apparatus, since it provides accurate temperaturecontrol, while also extending the life of heating element components. Inaddition, the present invention also provides a simple, yet accuratesystem for time compensation during a cook mode.

The foregoing description is a specific embodiment of the presentinvention. It should be appreciated that this embodiment is describedfor purposes of illustration only and that numerous alterations andmodifications may be practiced by those skilled in the art withoutdeparting from the spirit and scope of the invention.

The invention claimed is:
 1. A method of operating a control system to classify a medium in a cooking chamber as air or non-air comprising:determining a first measured temperature of the medium; storing the first measured temperature of the medium; activating a heating element to heat said medium for a first predetermined period of time; deactivating said heating element for a second predetermined period of time; determining a second measured temperature of the medium; comparing the first measured temperature of the medium to said second measured temperature; classifying the medium as air if said second measured temperature exceeds said first measured temperature by a predetermined amount; and controlling the heating element as a function of said classification.
 2. A method of operating a control system to control the temperature of a medium in a cooking chamber, said method comprising:activating a heating element to heat said medium; monitoring the temperature of said medium; determining if the temperature of said medium stops rising within a predetermined range of temperatures; and controlling said heating element to maintain the medium at a temperature below the temperature at which said medium stopped rising.
 3. The method of operating a control system as set forth in claim 2, wherein the temperature at which said medium is maintained is outside the predetermined range of temperatures.
 4. The method of operating a control system as set forth in claim 2, wherein the predetermined range of temperatures corresponds to the boiling point of water over normal altitude variations.
 5. The method of operating a control system as set forth in claim 4, wherein the predetermined range of temperatures is approximately 200° F.-220° F.
 6. A method of operating a control system to classify a medium in a cooking chamber as air, liquid or solid comprising;activating a heating element to heat said medium such that the temperature of said medium rises a first preprogrammed amount; measuring the time for the temperature to rise said first preprogrammed amount; deactivating said heating element; measuring the time for the medium temperature to fall a second preprogrammed amount; and classifying said medium as air, liquid or solid based upon the total time for the temperature of the medium to rise and fall.
 7. A method of operating a control system as defined in claim 6, wherein said system includes means for storing in memory the thermal characteristics of liquid and solid mediums and air, and means for calculating the thermal characteristics of the medium in said cooking chamber from the total time for the temperature of the medium to rise and fall.
 8. A control system for controlling the temperature of a medium in a cooking chamber comprising:a heater for applying heat to the medium; a temperature sensor for sensing temperature of the medium; and a controller operatively connected to the heater and the temperature sensor for controlling the amount of heat applied to the medium in two or more modes of operation, wherein in one mode of operation the controller determines if the temperature of the medium stops rising at a temperature indicative of the boiling point of water and automatically changes to another mode of operation wherein the heater is controlled to maintain said medium at a temperature below the temperature indicative of the boiling point of water. 